1. Field of the Invention
The present invention relates to an improved process for the delayed coking of heavy residual hydrocarbons that reduces the coking induction period and thereby enhances the coking process.
2. Description of Related Art
A coking unit is an oil refinery processing unit that converts the low value residual oil, or residua, from the vacuum distillation column or the atmospheric distillation column into low molecular weight hydrocarbon gases, naphtha, light and heavy gas oils, and petroleum coke. The process thermally cracks the long chain hydrocarbon molecules in the residual oil feed into shorter chain molecules. Coking is the preferred option for processing vacuum residues containing high level of metals because metals end up in the coke by-product and are disposed of more easily and economically in this solid form. The liquid coker products are almost free of metals. The processing of heavy crude oils having high metals and sulfur content is increasing in many refineries, and as a result the coking operations are of increasing importance to refiners. The increasing concern for minimizing air pollution is another incentive for treating vacuum residues in a coker, since the coker produces gases and liquids having sulfur in a form that can be relatively easily removed from the product stream.
The most commonly used coking unit is a delayed unit, or a “delayed coker”. In a basic delayed coking process, fresh feedstock is introduced into the lower part of a fractionator. The fractionator bottoms including heavy recycle material and fresh feedstock are passed to a furnace and heated to a coking temperature. The hot feed then goes to a coke drum maintained at coking conditions where the feed is cracked to form light products while heavy free radical molecules form heavier polynuclear aromatic compounds, which are referred to as “coke.” With a short residence time in the furnace, coking of the feed is thereby “delayed” until it is discharged into a coking drum. The volatile components are recovered as coker vapor and returned to the fractionator, and coke is deposited on the interior of the drum. When the coke drum is full of coke, the feed is switched to another drum and the full drum is cooled and emptied by conventional methods, such as by hydraulic means or by mechanical means.
Typical coking unit feedstocks are vacuum residues derived from fossil fuels. Selected properties and characteristics of vacuum residue samples derived from crude oils from the various geographical regions indicated are shown in Table 1. As can be seen from Table 1, vacuum residues have low American Petroleum Institute (API) gravities in the range of from 1 to 20 degrees and a sulfur content that ranges from 0.2 to 7.7 W %. In addition, vacuum residues are rich in nitrogen and can contain metals such as nickel and vanadium in relatively high concentrations which make them difficult to process in other refinery unit operations.
TABLE 1TachingBrentKirkukSafaniyaAthabascaBoscanRospomareSpecific Gravity0.9320.9841.0211.041.0381.0351.065API Gravity20.312.37.14.64.85.21.4Viscosity @ 100° F.1753808704000130040003500Sulfur0.21.65.25.44.95.67.67Nitrogen3800470040004300570078004200Conradson Carbon9.416.51824.616.719.326.3Residue (CCR)C5-Insolubles0.83.515.723.617.923.235.2C7-Insolubles0.317.713.610.214.123.9Nickel (Ni) ppmv1011524410112171Vanadium (V) ppmv7381251622801330278Ni + V ppmv17491772063811451349
Vacuum residues also contain asphaltenes in the range 0.3 to 35 W %, depending upon the source of the crude oil. Asphaltenes are defined as the particles precipitated by addition of a low-boiling paraffin solvent such as normal-pentane. It is commonly accepted that asphaltenes exist in solution in the petroleum. Asphaltenes are commonly modeled as a colloid, with asphaltenes as the dispersed phase and maltenes as the continuous phase. Petroleum residua can be modeled as ordered systems of polar asphaltenes dispersed in a lower polarity solvent phase, and held together by resins of intermediate polarity.
As schematically illustrated in FIG. 1, it is known to the prior art that asphaltenes are dispersed by resin molecules, or maltenes, while small molecules such as aromatics act as a solvent for the asphaltenes-resin dispersion and hydrocarbon saturates act as a non-solvent. If crude oil is separated into fractions and then mixed together with less resin content, asphaltenes will only be present as flocculates in solution. Addition of the maltenes or resins brings the asphaltenes back into solution until the equilibrium is disturbed by addition of hydrocarbon saturates, in which case asphaltenes will again start to flocculate.
It is well known and accepted that coke formation is delayed when the asphaltenes are in solution in the petroleum. This delay in coke formation is also referred as the “induction period” which immediately precedes the formation of coke. During this period, valuable lighter components and/or secondary products formed by coking of feedstocks are subject to continued thermal cracking and recombine to form undesirable high molecular weight polymeric compounds.
It is also known from independent studies of the thermal cracking of bitumens that the yield of gaseous products increases with the residence time in the coking unit and that liquid yields are correspondingly reduced.
It is also desirable to produce a coke having a volatile matter content of not more than about 15 W %, and preferably in the range of 6 to 12 W %.
It is therefore an object of this invention to address the problem of how to reduce the coking induction period so that the residence time of the feed in the coke drum is shortened. This will maximize the desired yield of liquids and minimize the coke yield.
As used herein, the terms “coking unit” and “coker” refer to the same apparatus, and are used interchangeably.